In an automatic transmission for a vehicle, a frictional engagement element (a clutch) having a plurality of friction plates is disposed on a transmission path of a rotation driving force inputted from an engine.
In this frictional engagement element, inner diameter side friction plates and outer diameter side friction plates, both of the inner diameter side and the outer diameter side friction plates being relatively rotatably disposed on the same axis, are alternately arranged in a direction of a rotation axis. When the inner diameter side friction plates and the outer diameter side friction plates are pressed by a piston in the direction of rotation axis, facing members disposed on each of the inner diameter side friction plates are brought into contact with a corresponding one of the outer diameter side friction plates under pressure. Thus, the inner diameter side friction plates and the outer diameter side friction plates are engaged in a state of being incapable of relative rotation.
In the frictional engagement element, when the relative rotation between each of the inner diameter side friction plates and the corresponding one of the outer diameter side friction plates is regulated, a transmission of the rotation driving force between an upstream side of the frictional engagement element and a downstream side of the frictional engagement element becomes possible. When the relative rotation becomes possible, the transmission of the rotation driving force cannot be carried out.
Therefore, in the automatic transmission for the vehicle, transmission/non-transmission of the rotation driving force in a plurality of the frictional engagement elements equipped in the automatic transmission is modified in accordance with a traveling state of the vehicle so that a transmission path of the rotation driving force is switched to achieve a desired traveling performance and a desired gearshift stage.
It should, herein, be noted that, when the transmission/non-transmission of the rotation driving force in each of the frictional engagement elements is switched, a state of the friction plates in a corresponding one of the frictional engagement elements is switched between an engagement state and a release state so that the inner diameter side friction plates and the outer diameter side friction plates become temporarily in a slipped state and generate heat.
Therefore, in the automatic transmission for the vehicle, lubricating oil supplied from an inner diameter side of the automatic transmission is guided to the clutch according to a centrifugal force and the inner diameter side friction plates and the outer diameter side friction plates are cooled with the guided lubricating oil. For example, a Patent Document 1 discloses that oil grooves are disposed on the respective facing members installed on each of the inner diameter side friction plates to improve a coolability of the clutch.